Russia's space agency has so far failed to contact its $13-million unmanned Phobos-Grunt Mars probe, which it lost contact with immediately after launch early on Wednesday -- and may end up as the 18th in a long string of unsuccessful Russian missions to the Red Planet.

"So far all efforts to communicate with the craft have been unsuccessful," lead mission scientist Alexander Zakharov of Moscow's Space Research Institute told Reuters.

The probe is stuck in a dangerously low orbit, officials said, and if uncontacted could eventually crash down to Earth. Should the craft fail, it will join a surprisingly long string of Russian missions to Mars that have failed, even as U.S. rovers have logged hundreds of hours on the Red Planet. Meanwhile, NASA plans to launch a $2.5 billion rover designed to assess the planet's suitability for life later this month, toward the end of a launch window for Mars flights that comes every 780 days.

If Phobos-Grunt eventually crashes back to Earth, its cargo may become a problem: The craft carries a full payload of toxic hydrazine and nitrogen tetroxide fuel and small cargo of radioactive cobalt-57.

It is unclear how much of it will survive the fiery plunge through the atmosphere.